1. Field of the Invention
The present invention relates to a reflection type LCD (liquid crystal display) device and a transmission/reflection type LCD device for use in an OA (office automation) apparatus (such as a word processor or a personal computer), a portable information apparatus (such as an electronic organizer), a camera and VTR unit which includes a liquid crystal monitor, and the like.
2. Description of the Related Art
Unlike a CRT (cathode ray tube) or EL (electroluminescence) display device, an LC (liquid crystal) panel does not itself emit light. A transmission type LCD device includes a back light (a device including a fluorescent lamp) provided on the back of the LC panel so that a display can be produced by controlling the LC panel to selectively transmit and block light from the back light.
In a transmission type LCD device, however, the power consumed by the back light typically accounts for 50% or more of the total power consumption. Therefore, the power consumption of the LCD device substantially increases by the provision of a back light.
A reflection type LCD device is preferably used in a portable information apparatus which is carried around by the user and often used outdoors. A reflection type LCD device includes a reflector, instead of a back light, so that a display can be produced by controlling the LC panel to selectively transmit and block ambient light which is reflected by the reflector.
For example, Japanese Laid-Open Publication No. 5-323371 discloses a reflection type LCD device in which an interlayer insulating layer is provided over a plurality of gate lines, a plurality of source lines perpendicularly crossing the gate lines, and switching elements each connected to one of the gate lines and one of the source lines, so that a pixel region can be extended over the lines to improve the aperture ratio. A reflection electrode region of a pixel electrode formed on the interlayer insulating layer overlaps the gate lines and the source lines via the interlayer insulating film therebetween. Moreover, concave/convex portions of a few .mu.m height/depth are provided on the surface of the interlayer insulating layer, thereby providing a reflection electrode region whose surface exhibits a concave/convex profile. In such an LCD device having such a structure, ambient light of various incident angles can be used as display light, thereby producing a desirable display with little viewing angle dependency.
However, in the conventional LCD device as disclosed in Japanese Laid-Open Publication No. 5-323371, the gap between the pair of substrates of the LCD device (so called a "cell gap") cannot be accurately controlled, thereby deteriorating the display quality.
In the conventional LCD device, particulate spacers are provided between the pair of substrates (e.g., an active matrix substrate and a counter substrate) interposing an LC layer therebetween so as to control the cell gap. More specifically, particulate spacers (e.g., spherical spacers made of a plastic resin) are uniformly dispersed on at least one of the substrates, after which the substrates are pressed onto each other via the particulate spacers so as to be attached to each other with a sealant therebetween, thus controlling the gap between the pair of substrates. The size of the spacers, which dictates the cell gap, can be determined based on the liquid crystal display mode to be employed. A spherical spacer (or a cylindrical spacer) having a diameter of about 2-6 .mu.m is typically used to provide a cell gap with which an optimal optical path length is obtained. With the method as described above in which particulate spacers are dispersed, it is difficult to place the spacers at desired positions on the substrate, whereby a uniform cell gap may not be obtained in a case where there are concave/convex portions (steps) provided on the surface of the substrate.
An exemplary method for forming such concave/convex portions on the surface of the interlayer insulating layer is as follows. First, a positive type photosensitive resin is applied on a substrate using a spin coater, or the like. Then, the substrate is exposed via a mask and developed. The mask includes holes arranged in a predetermined pattern corresponding to reflection electrode regions. Then, the substrate is subjected to a heat treatment so as to form the concave/convex portions. The present inventors have found that the following problem arises when the interlayer insulating layer having concave/convex portions is provided only for the reflection electrode region, aiming to merely improve the efficiency in using reflected light.
The problem will be described below with reference to FIGS. 17A and 17B. When providing an underlying layer using a positive type photosensitive resin to provide a concave/convex profile to the surface of the interlayer insulating layer, if a portion of the underlying layer corresponding to the non-display region is left unexposed, a defective display may occur along the periphery of the display region adjacent the non-display region (i.e., the portion of the display region hatched in FIG. 17A).
The unexposed portion of the underlying layer (which corresponds to the non-display region) has a larger thickness than the exposed portion (which corresponds to the display region where convex portions are formed) due to the reduction in thickness through an exposure/development process for the positive type resist. Then, if spacers of the same diameter are dispersed in the cell gap, the cell gap may vary between the display region and the non-display region. More particularly, a portion of the display region will have a cell gap larger than a predetermined cell gap. The variation in cell gap may cause the defective display.
Particularly, the present inventors have found that the deterioration in display quality due to the variation in cell gap is more significant in a transmission/reflection type LCD device (as that disclosed in commonly-owned Japanese Laid-Open Publication No. 9-201176) than in a reflection type LCD device. In order to improve the display quality of a transmission/reflection type LCD device, it is preferred to control the optical path length of the reflection region to be as close as possible to that of the transmission region. In particular, the thickness of the LC layer in the reflection region is preferably about 1/2 of that in the transmission region. If the cell gap is shifted from the optimal value by X .mu.m, for example, the optical path length of the transmission region will be shifted from the optimal value by X .mu.m, while that of the reflection region will be shifted from the optimal value by 2.times. .mu.m. Thus, display quality deteriorates due to the difference in the influence of the cell gap shift on the display between the transmission region and the reflection region.
The variation in cell gap deteriorates the display quality in any of the various LC display modes, including the TN (twisted nematic) mode, the parallel alignment mode, the vertical alignment mode, etc. Particularly, when a normally white mode is employed as the display mode, a change in cell gap substantially reduces the contrast ratio. In a normally white mode, a black display is produced in the presence of a predetermined applied voltage. Therefore, if there is a variation in cell gap, the voltage required to produce a black display will vary for different positions, thereby deteriorating the quality of a black display. The contrast ratio of an LCD device is more influenced by the black display quality than by the white display quality. Thus, the deterioration in contrast ratio is more significant in a normally white mode than in a normally black mode.